Microbial Metabolism, Growth, Control, and Antimicrobial Drugs – Study Guide

Microbial Metabolism

Catabolism vs. Anabolism

Microbial metabolism encompasses all chemical reactions that occur within a microbe. These reactions are divided into two main categories:

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. Example: Glycolysis.

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. Example: Protein synthesis.

Types of Microbial Nutrition

• Chemoheterotroph: Organisms that obtain both energy and carbon from organic compounds. Most bacteria, fungi, and animals are chemoheterotrophs.

• Chemoautotroph: Organisms that obtain energy from inorganic compounds and carbon from CO2. Example: Nitrifying bacteria.

• Photoheterotroph: Organisms that use light for energy but require organic compounds as a carbon source. Example: Some purple non-sulfur bacteria.

• Photoautotroph: Organisms that use light for energy and CO2 as a carbon source. Example: Cyanobacteria, plants.

Enzymes: Properties and Function

• Structure: Enzymes are typically globular proteins with an active site where substrates bind.

• Factors Influencing Enzyme Activity:

  • Temperature

  • pH

  • Substrate concentration

  • Presence of inhibitors or activators

• Function: Enzymes act as biological catalysts, speeding up reactions by lowering activation energy.

• Types of Enzyme Inhibition:

  • Competitive inhibition: Inhibitor binds to the active site, blocking substrate.

  • Noncompetitive inhibition: Inhibitor binds elsewhere, changing enzyme shape and function.

Cellular Respiration vs. Fermentation

• Cellular Respiration: Involves glycolysis, Krebs cycle, and electron transport chain. Final electron acceptor is usually oxygen (aerobic) or another inorganic molecule (anaerobic).

• Fermentation: Incomplete oxidation of glucose; final electron acceptor is an organic molecule. Produces less ATP than respiration.

Oxidase Test

• Purpose: Detects the presence of cytochrome c oxidase enzyme in bacteria, used to differentiate among bacterial species.

Lipid and Protein Catabolism

• Lipid Catabolism: Lipases break down lipids into fatty acids and glycerol. Fatty acids enter β-oxidation to generate acetyl-CoA for the Krebs cycle.

• Protein Catabolism: Proteases break down proteins into amino acids, which are deaminated and enter metabolic pathways as intermediates.

Importance of Microbial Metabolism

• Understanding metabolism helps in identifying microbes, developing antibiotics, and controlling microbial growth in clinical and industrial settings.

Microbial Growth

Growth Curve Phases

Bacterial growth in a closed system follows a predictable pattern:

• Lag Phase: Cells adapt to environment; no increase in number.

• Log (Exponential) Phase: Rapid cell division; population doubles at a constant rate.

• Stationary Phase: Growth rate slows; nutrients deplete, waste accumulates; cell death equals cell division.

• Death Phase: Cells die at an exponential rate due to lack of nutrients and toxic waste buildup.

Clinical Importance: The log phase is most susceptible to antibiotics; stationary phase is important for survival and persistence of pathogens.

Oxygen Requirements for Growth

• Obligate Aerobes: Require oxygen; use aerobic respiration.

• Facultative Anaerobes: Grow with or without oxygen; use aerobic respiration, anaerobic respiration, or fermentation.

• Obligate Anaerobes: Cannot tolerate oxygen; use anaerobic respiration or fermentation.

• Aerotolerant Anaerobes: Do not use oxygen but tolerate its presence; use fermentation.

• Microaerophiles: Require low levels of oxygen.

Protective Enzymes: Superoxide dismutase (SOD), catalase, and peroxidase detoxify reactive oxygen species. Obligate aerobes and facultative anaerobes possess these enzymes.

Temperature Requirements

• Psychrophiles: Grow best at 0–15°C.

• Mesophiles: Grow best at 20–45°C (human pathogens are mesophiles).

• Thermophiles: Grow best at 55–80°C.

• Hyperthermophiles: Grow above 80°C.

pH Requirements

• Acidophiles: Grow best at pH < 5.5.

• Neutrophiles: Grow best at pH 5.5–8.0.

• Alkaliphiles: Grow best at pH > 8.0.

Halophiles and Osmosis

• Obligate Halophiles: Require high salt concentrations for growth.

• Facultative Halophiles: Can tolerate high salt but do not require it.

• Relation to Osmosis: High salt environments cause water to leave cells (osmosis); halophiles have adaptations to prevent dehydration.

Control of Microbial Growth

Terminology

• Sterilization: Destruction of all microbial life, including spores.

• Disinfection: Destruction of vegetative pathogens on inanimate objects.

• Antisepsis: Destruction of pathogens on living tissue.

• Sanitization: Lowering microbial counts to safe public health levels.

-cidal vs. -static Treatments

• -cidal: Treatments that kill microbes (e.g., bactericidal).

• -static: Treatments that inhibit growth without killing (e.g., bacteriostatic).

Physical Control Methods

• Heat: Moist heat (autoclaving, boiling) and dry heat (oven) denature proteins and kill microbes.

• Filtration: Physically removes microbes from liquids or air using filters.

Chemical Control Methods

• Alcohols: Denature proteins and disrupt membranes (e.g., ethanol, isopropanol).

• Halogens: Oxidize cellular components (e.g., chlorine, iodine).

Antimicrobial Drugs

Bacteriostatic vs. Bactericidal

• Bacteriostatic: Inhibit bacterial growth; rely on host immune system to eliminate infection.

• Bactericidal: Kill bacteria directly.

Broad-Spectrum vs. Narrow-Spectrum Drugs

• Broad-Spectrum: Effective against a wide range of microbes (e.g., both Gram-positive and Gram-negative bacteria).

• Narrow-Spectrum: Effective against specific groups of microbes.

Superinfections

• Significance: Occur when normal microbiota are destroyed by antibiotics, allowing opportunistic pathogens to proliferate (e.g., Clostridioides difficile infection after broad-spectrum antibiotic use).

Dosage and Route of Administration

• Dosage: Must achieve effective concentration without causing toxicity.

• Route: Oral, intravenous, or topical; affects drug absorption and effectiveness.

Drug Interactions

• Positive Interactions (Synergism): Combined drugs have enhanced effect.

• Negative Interactions (Antagonism): One drug reduces the effectiveness of another.

Mechanisms of Action of Antibacterial Drugs

• Inhibition of Cell Wall Biosynthesis: e.g., β-lactams (penicillins), vancomycin.

• Inhibition of Protein Synthesis: e.g., tetracyclines, macrolides.

• Disruption of Membrane Function: e.g., polymyxins.

• Inhibition of Nucleic Acid Synthesis: e.g., quinolones, rifamycins.

• Inhibition of Metabolic Pathways: e.g., sulfonamides (folic acid synthesis inhibitors).

Modes of Action: Antifungal, Antiprotozoan, Antihelminthic, and Antiviral Drugs

• Antifungal: Target ergosterol in fungal membranes (e.g., amphotericin B).

• Antiprotozoan: Interfere with DNA replication or metabolic pathways (e.g., metronidazole).

• Antihelminthic: Disrupt helminth metabolism or neuromuscular function (e.g., mebendazole).

• Antiviral: Inhibit viral entry, replication, or release (e.g., acyclovir, oseltamivir).

Development and Acquisition of Drug Resistance

• Mechanisms:

  • Enzymatic drug inactivation (e.g., β-lactamases)

  • Alteration of drug targets

  • Decreased permeability or increased efflux of drug

  • Bypass of metabolic pathways

• Genetic Processes:

  • Transformation: Uptake of free DNA from environment.

  • Transduction: Transfer of DNA by bacteriophages.

  • Conjugation: Direct transfer of DNA between bacteria via pilus.

Summary Table: Mechanisms of Antimicrobial Drug Resistance

Mechanism	Description	Example
Enzymatic Inactivation	Microbe produces enzymes that destroy or modify the drug	β-lactamase inactivating penicillins
Altered Target	Drug target is modified so drug cannot bind effectively	Altered ribosomal proteins for macrolide resistance
Efflux Pumps	Microbe pumps drug out of the cell	Tetracycline efflux pumps
Decreased Permeability	Changes in membrane proteins reduce drug entry	Porin mutations in Gram-negative bacteria
Bypass Pathways	Microbe uses alternative metabolic pathways	Folate synthesis bypass in sulfonamide resistance

Additional info: Understanding these concepts is essential for clinical decision-making, infection control, and the development of new antimicrobial strategies.